1. Field of the Invention
The present invention relates to an optical semiconductor device and a fabricating method thereof, in particular, to an optical semiconductor device that has a low device capacitance that allows the device to modulate directly at a high frequency of 10 Gb/s or higher and a fabricating method thereof.
2. Description of the Related Art
As demand of information communication increases, technologies that allow much information to be transmitted at low cost are required for not only a trunk line, but a branch line. Particularly, in recent years, as the Internet is becoming common, technologies allowing digital contents of audio data and video data of large capacities to be transmitted to end users at high speed and at low cost are demanded.
For example, it is predicted that optical communications will be performed in not only LAN systems of companies, but terminal units of home servers. In near future, it is expected that the market of optical communication systems used in short distance and provided at low cost will grow at an explosive pace. Thus, needs for a light source that allows such system to directly perform a modulating operation at high speed and at low cost are becoming strong.
Conventionally, a system that directly modulates a semiconductor laser is used in a short-distance low-cost communication. To directly modulate a semiconductor laser, it is preferred to decrease the parasitic capacitance and series resistance of a device.
In addition, characteristics of a semiconductor laser are largely varied with temperature. To stably operate a system, the temperature of the semiconductor laser is controlled in a constant level by a Peltier cooler. However, a more inexpensive system requires a semiconductor laser that does not need a temperature control and does not have a Peltier cooler.
To operate a semiconductor laser without cooling it, a lower threshold current, a structure free of a leak current, and an improved light emission efficiency are desired. To do that, a semiconductor laser having a buried hetero structure (a buried hetero semiconductor laser) is used.
FIG. 16 is a sectional view showing an example of the structure of a existing buried hetero semiconductor laser. Referring to FIG. 16, an n-type InP buffer layer 52 and an InGaAsP-MQW (a multiple quantum well) active layer 53 are successively grown over an n-type InP substrate 51. The InGaAsP-MQW active layer 53 is formed in a stripe whose width is 1.5 xcexcm and that has a trapezoidal section. The InGaAsP-MQW active layer 53 is surrounded by a p-type InP burying layer 54 and an n-type InP burying layer 55. The active layer 53 is buried in these burying layers.
A p-type InP cladding layer 56 and a p-type InGaAs contact layer 57 are successively formed on the n-type InP burying layer 55. In addition, to electrically isolate the burying layers, grooves 58 are formed on both sides of the active layer 53. A SiO2 film 59 is formed in the groove 58. The distance between two grooves 58 formed on both the sides of the active layer 53 is 10 xcexcm.
A p-side electrode 60 and a bonding pad 61 are successively formed on the p-type InGaAs contact layer 57. An n-side electrode 62 is formed on the rear surface of the n-type InP substrate 51.
In such a semiconductor laser, since p-n inverse junctions of an n-type InP layer and a p-type InP layer are formed on both sides of the active layer 53. Thus, a leak current does not flow. A current injected from the p-side electrode 60 flows in only the InGaAsP-MQW active layer 53. When a plus voltage is applied to the p-side electrode 60, a bias voltage is also applied to a p-n junction of the burying layers. As a result, a depletion layer takes place at the interface of the n-type InP layer 55 and the p-type InP layer 54. The depletion layer that takes place in the burying layer becomes an electrical capacitor.
In such a buried hetero semiconductor laser, a current injected to an electrode effectively flows in only the active layer. Thus, the buried hetero semiconductor laser has the advantage of having a low threshold current.
As another type of a semiconductor laser, a ridge waveguide semiconductor laser is also known. In the ridge waveguide semiconductor laser, a ridge stripe shaped cladding layer is formed on an active layer. A current injected and supplied from an electrode is guided to the ridge portion so as to confine the current. In the ridge portion that is a current confinement layer, the width of the ridge stripe adjacent to the active layer is for example 2.5 xcexcm.
The ridge waveguide semiconductor laser has an advantage of no parasitic capacitance of a depletion layer because of no layers unlike with the forgoing buried hetero semiconductor laser. However, the ridge waveguide semiconductor laser has a disadvantage of which a current tends to flow in a peripheral portion of an active layer adjacent to a ridge stripe, and a threshold current is higher.
As described above, in the buried hetero semiconductor laser shown in FIG. 16, a low oscillation threshold current can be easily accomplished. However, to perform a high-speed modulation, it is necessary to decrease the parasitic capacitance. To obtain a response of 10 GHz or higher, the parasitic capacitance should be 3 pF or lower.
FIG. 17 shows the relation between the width of the p-n junction portion of burying layers (hereinafter referred to as a mesa width) and the parasitic capacitance thereof. As denoted by a solid line in FIG. 17, to decrease the parasitic capacitance to 3 pF or lower, it is necessary to decrease the mesa width of the burying layers to 4.8 xcexcm or lower.
FIG. 18 shows a buried hetero semiconductor laser with a narrowed mesa width. In the structure, when the mesa width that is the width of the p-n junction portion of the burying layer is 4.8 xcexcm, the width of a mesa top portion MT is 3 xcexcm. It was very difficult to form a p-side electrode and so forth on the narrow mesa top portion MT.
Even if such an electrode is formed, the width of the electrode will become as large as around 1 xcexcm. Thus, the series resistance of the wiring portion will become large. As a result, the time constant (proportional to the product of capacitance and series resistance) cannot be reduced.
Moreover, in the buried hetero semiconductor device shown in FIG. 16, when the carrier concentration of the n-type InP burying layer 55 is lowered, the width of the depletion layer that takes place with a bias voltage applied to the p-n junction can be increased. The parasitic capacitance can be reduced without the narrowed mesa width.
A broken line shown in FIG. 17 represents the relation between the parasitic capacitance and the mesa width in the case that the carrier concentration of the n-type InP burying layer 55 is 1xc3x971017 cmxe2x88x923. As is clear from the diagram, when the carrier concentration is 1xc3x971017 cmxe2x88x923, even if the mesa width is 10 xcexcm, the parasitic capacitance of the junction portion is 3 pF or lower.
However, in such a structure, the leak current increases and thereby the oscillation threshold current increases. For example, when the carrier concentration is 1xc3x971018 cmxe2x88x923, the threshold current is 6 mA. In contrast, when the carrier concentration is 1xc3x971017 cmxe2x88x923, the threshold current becomes 9 mA. In other words, the threshold current adversely increases by 50%.
Additionally, since the ridge waveguide semiconductor laser does not have burying layers, the device parasitic capacitance may become around 1 pF. However, since the optimum width of the active layer for the fundamental transverse mode oscillation is as large as around 2.5 xcexcm, the oscillation threshold current becomes high (for example, around 11 mA). Further, since the active layer of the ridge waveguide semiconductor laser is a little wider than that of the buried hetero semiconductor laser, after the oscillation is started, the carrier density becomes slightly low. Thus, it is relatively difficult to increase the relaxation oscillation frequency.
An object of the present invention is to provide an optical semiconductor device that features a low threshold current and easiness of a single transverse mode oscillation and that has a low parasitic capacitance that allows a direct modulation at high speed; and a fabricating method thereof.
An optical semiconductor device according to an embodiment of the present invention comprises a first conduction type substrate, an active layer formed in a stripe on the first conduction type substrate, a mesa shaped burying layer formed around the active layer and having a larger band gap than that of the active layer, and a groove that electrically isolates the burying layer, wherein the section of the burying layer is in an inverse trapezoid shape of which the upper base side is longer than the lower base side.
An optical semiconductor device according to another embodiment of the present invention comprises a first conduction type substrate, an active layer formed on the first conduction type substrate, a current confinement layer formed in a ridge stripe on the active layer, a groove that electrically isolates the current confinement layer, the groove having a space portion, and a floating wiring portion formed over the groove.
An optical semiconductor device according to another embodiment of the present invention comprises a first conduction type substrate, an active layer formed on the first conduction type substrate, and a mesa shaped current confinement layer formed on the active layer, wherein a floating wiring portion is formed around the current confinement layer.
A method for fabricating an optical semiconductor device according to another embodiment of the present invention comprises preparing a first conduction type substrate, forming an active layer in a stripe on the first conduction type substrate, forming a burying layer around the active layer, the burying layer having a band gap larger than that of the active layer, forming grooves that isolate the burying layer in a stripe so as to form the section of the burying layer in an inverse trapezoid shape of which the upper base side is longer than the lower base side, forming a resin layer filled in the groove, forming wiring on the resin layer, and selectively etching the resin layer so as to remove all or part of the resin layer and form a space portion in the groove below the wiring.
A method for fabricating an optical semiconductor device according to another embodiment of the present invention comprises preparing a first conduction type substrate, forming an active layer on the first conduction type substrate, forming a current confinement layer on the active layer, forming a resin layer filled around the current confinement layer, forming wiring on the resin layer, and selectively etching the resin layer so as to remove all or part of the resin layer and form a space portion below the wiring.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of embodiments thereof, as illustrated in the accompanying drawings.